A certain car can travel 40 minutes on a gallon of gasoline at 60 miles per hour. If the car had started with a full tank and had 8 gallons of gasoline left in its tank at the end, then what percent of the tank was used to travel 80 miles at 60 mph?
A. 15% B. 20% C. 25% D. 30% E. 40%
(B) 60 miles per hour is a mile a minute. 80 miles at 60 miles per hour will take 80 minutes. Since 40 minutes is 1 gallon, 80 minutes will be 2 gallons. The full tank holds 2 + 8 = 10 gallons. 2 gal/10 gal = 2/10, which is 20%. The correct answer is choice (B). 
In this question, how do you know that the full tank is 10 gallons?
